1. Field of the Invention
This invention relates to automatic direction finding systems and, ore particularly, to an automatic direction finding system utilizing the doppler principle by which an antenna is electrically circulated around a circular path thereby frequency modulating the transmitted signal such that the phase of the modulating signal indicates the bearing of the radio transmission.
2. Description of the Prior Art
Radio direction finding has long been used by aircraft and marine services as an aid for location and navigation purposes. Most radio direction finding systems utilize some form of highly directive antenna. In these systems, directional information is obtained by relative amplitude comparisons as the antenna is rotated. The gain patterns for most of these antennas include a peak and a null, but since the null response is generally much narrower than the peak response the null response is preferred since it is capable of providing more accurate directional information. Earlier systems required manual rotation of the antenna and considerable operator skill to avoid erroneous results. With conventional systems, rotation of the antenna is normally simulated by vectorially resolving the outputs of two directional antennas mounted in quadrature relationship to one another. An omnidirectional antenna can be combined with the two directional antennas to generate an antenna having a cardioid pattern. Under optimum conditions, these conventional audio direction finding systems can provide satisfactory results. However, under less than optimum conditions a number of problems develop. Where these conventional systems are used near reflective land masses or other obstruction, the incoming signal is reflected by the land mass or obstruction so that a plurality of signals arriving at different bearings are received by the direction finding antenna. This condition, commonly called multipath, effectively distorts the sensitivity pattern of the direction finding antenna often producing several relatively indistinct nulls. The results under these conditions are generally both confusing and ambiguous.
Another problem associated with conventional automatic direction finding systems utilizing a null antenna occurs under weak signal conditions. In these circumstances, as the null in the pattern is approached the signal drops beneath the sensitivity of the receiver thereby effectively broadening the width of the null and consequently limiting the resolution of the system on weak signals. Another problem associated with this type of conventional system where the antenna is rotating at an audio frequency is that the amplitude variations in the antenna pattern amplitude modulate the incoming signal thereby injecting an audio tone in the received signal. This is particularly troublesome during search and rescue operations where identification and bearing are both important.
Another variety of automatic direction finding system which utilizes the doppler principle avoids many of the above mentioned problems associated with conventional systems utilizing null antennas. In automatic direction finding systems utilizing the doppler principle, a single receiver antenna circulates at a constant speed along a circular path. As the antenna approaches the source of the received signal, the apparent frequency of the received signal is increased, and when the receiver antenna moves away from the source of the received signal, the apparent frequency of the received signal is decreased. Where the frequency of the received signal is equal to the average frequency of the signal, the antenna is at its closest and farthest distances from the source of the received signal. By noting the position of the antenna as the frequency of the received signal crosses over from an above average frequency to a below average frequency, the bearing of the radio transmission can be determined. Practical designs for doppler automatic direction finding systems do not mechanically rotate the antenna since the velocity required to place the doppler component above the communication audio spectrum is quite large. For example, a velocity of 240,000 revolutions per minute is required to generate a doppler component of 4Khz. Instead of mechanically rotating the antenna, practical systems utilize a circular fixed array of vertical antenna elements. The doppler modulation signal is provided by sequentially connecting the receiver to successive antenna elements, generally by means of a capacity switch. It has been universally assumed in the past that many antenna elements were required in order to approximate a single antenna moving along a circular path. Decreasing the number of antenna elements, it was reasoned, would seriously decrease system accuracy. As a result of this reasoning, conventional doppler systems contain a relatively large number of antenna elements which materially increases the cost and complexity of such systems. Despite the disadvantages of cost and complexity of these conventional doppler systems, they have been relatively successful in eliminating the aforementioned problems associated with systems using a null antenna. Since the field pattern of the doppler antenna is relatively omni-directional, the received signal is not amplitude modulated by the electrical rotation of the antenna. The multipath problem is eliminated by the "capture effect" of FM receivers which are used in connection with the doppler antenna. Under most conditions, the direct wave is stronger than the deflected or reflective waves. The FM receiver locks onto the strongest signal and suppresses the weaker signals. Thus conventional doppler systems are somewhat more expensive and complex than null systems but they provide superior results.